The number 6 has a special significance. It represents the Law and Order and also associated with a Karmic Law of cause and effect. Number 6 is a perfect number, symmetrical in all parts, the number of the Soul.
Plums are ready they are not hard as a rock, soft just a little when pressed. I wouldn't use windfall, they are too ripe. If your making jam, it is best to use some underripe with the ripe ones. Last season I added small diced candied ginger to the plum jam. Really turned out nice.
Fold the top of the bag down once or twice to secure it. Set the bag aside and keep it at room temperature for one or two days. Open the bag after two days and check for ripeness. Ripe red plums will be almost completely red and ripe black plums will be almost completely black.